Granitoids from West Kalimantan

This project aims to date metagranitoids and granitoids from the Upper Kualan district in West Kalimantan by U-Pb zircon LA-ICP-MS to study Triassic rocks in this region and to characterise Mesozoic magmatism in western Borneo. The results will help to reconstruct the Early Mesozoic tectonic history; i.e. if this region already belonged to the Sundaland margin in the Triassic, and therefore where the boundary between the Southwest Borneo block and the Malay Tin Belt (Triassic Sundaland) may be located.

West Kalimantan metagranitoids

Background

In West Kalimantan there are magmatic and metamorphic rocks which form the northwestern part of the Schwaner Mountains. Only limited dating has been carried out on the (meta-) igneous rocks in that region. The granitoids assigned to the Laur Granite and the Sepauk Tonalite were dated as Cretaceous (75 to 115 Ma) by K-Ar analysis (Haile et al., 1977; Bladon et al., 1989; Sanyoto & Pieters, 1993). The Sepauk Tonalite has been described as generally foliated by Sanyoto & Pieters (1993). Widespread metamorphic rocks in the northern zone were assigned to the Pinoh Metamorphics which were assumed to be Palaeozoic (Haile, 1974; van Bemmelen, 1949; Williams & Simamora, 1988) to Triassic in age (Sanyoto & Pieters, 1993). However, recent studies by the SE Asia Research Group revealed a Cretaceous age for the Pinoh Metamorphics (Davies, 2013; Davies et al., 2014).

The SW Borneo block is characterised by Cretaceous igneous and metamorphic rocks, as indicated by U-Pb zircon dating of Davies (2013) and Davies et al. (2014). Recently, one metatonalite sampled by Setiawan et al. (2013) from west of Pontianak has been dated as Triassic (233 ± 3 Ma) by zircon U-Pb LA-ICP-MS. The age is similar to ages reported by Williams et al. (1988) from northwest Kalimantan (c. 204 to 320 Ma), and for the Jagoi Granodiorite (c. 209 Ma) in West Sarawak (Breitfeld, 2015). The Embuoi Complex is exposed northeast of the study area. It is composed of igneous and metamorphic rocks which have been dated with the K-Ar method as Triassic (Bladon et al., 1989; Supriatna, 1989) and they indicate widespread Triassic magmatism and subsequent deformation. Breitfeld (2015) reported Late Triassic Ar-Ar ages from schists in West Sarawak which are interpreted to indicate a collision event.

The Triassic magmatism had been related to subduction in the Triassic beneath Sundaland
and interpreted to have been intruded in a Triassic accretionary margin in eastern Sundaland (Hall, 2012, 2013) at the Pacific margin. So far, similar rocks have not beenfound anywhere further east in the Schwaner Mountains. The Triassic rocks could indicate the southernmost extent of the Malay Tin Belt of Sundaland and the western limit of SW Borneo.